The present invention relates to a process for removing water from a hydrocarbon process stream. When processing hydrocarbon streams in processes such as isobutane alkylation in which the catalyst is sensitive to water, it is necessary to dry the hydrocarbon stream to minimise the effect of the water on the catalyst. Drying of a hydrocarbon stream is typically achieved by passing the stream over a solid absorbent such as alumina or molecular sieve. Subsequently, the absorbent is regenerated by heating it in a stream of inert gas to a temperature, which is substantially above the temperature of absorption i.e. 250–300° C. This method is effective for drying down to water levels below 10 ppm in the hydrocarbon stream. The disadvantage of this approach is that the absorption and the desorption is done under substantially different conditions in the same vessel. Consequently, the vessel has to be taken out of drying service for regeneration making the operation discontinuous. In addition the system including the adsorption vessel has to be designed for substantial changes in temperature and sometimes also in pressure to accommodate the different conditions for drying and regeneration.
Wash of hydrocarbon stream with a saturated solution of a highly soluble salt such as calcium chloride may be used for drying. This method does not dry the hydrocarbon stream efficiently.
Strong acids such as concentrated sulphuric acid are efficient drying agents, but such material is too reactive for many drying operations. Sulphuric acid may for instance act either as an oxidising agent oxidising the hydrocarbons or as acid catalyst catalysing undesirable reactions.
According to the present invention an improved process for the continuous drying of a hydrocarbon stream at a temperature being effective in drying the stream comprises contacting the stream with an ionic liquid drying agent comprising a salt of sulphuric acid being in liquid or melted form at the drying temperature.
Extraction of the hydrocarbon stream with a dry ionic liquid made from sulphuric acid by addition of up to one equivalent of one or more basic nitrogen compounds including amines, such as diethylmethyl amine or triethanolamine or a basic heterocyclic compound, e.g. 1-butyl pyrollidine, provides a convenient and efficient method for continuous drying of a hydrocarbon stream. The addition of the basic nitrogen compound lowers the activity of the sulphuric acid in such a way that undesired side reactions being a problem if sulphuric acid is used are avoided. Such undesired side reactions may for instance be oxidation or the hydrocarbons by the sulphuric acid acting as an oxidising agent or it may be acid catalysed reactions induced by the high acidity of sulphuric acid.
The extraction may be performed in a continuous counter-flow extraction operation in an extraction column or other liquid-liquid extraction equipment or it may be achieved using either a co-flow column or a simple mixer separator set. In the later embodiment of the invention a stream comprising the liquid sulphate drying agent phase and another stream of the hydrocarbon phase are contacted in a mixer, which may be a static mixed, and the two phases are subsequently separated again in a separator. The mixer-separator embodiment provides for a relatively inexpensive way to contact the two liquid phases.
The liquid sulphate drying agent may be continuously regenerated by drying in a subsequent step of stripping off the water at elevated temperature in a stripping column using an inert stripping agent such as a hydrocarbon or nitrogen or air. The thus regenerated drying agent is then recycled to the extraction step.